Heretofore, the following methods have been proposed for organic coating of the welded part, i.e. the exposed metal portion.
Firstly, a method has been proposed which comprises coating a liquid paint on the exposed metal portion by spraying or roll coating, and drying and curing it by heating thereby to coat the welded part. By this first method, it is difficult to form a coated film of a uniform thickness along the shape of the welded part, and it has the disadvantage that to prevent the exposure of the metal completely, the coated film undesirably thicker becomes in part. The thick portion tends to foam during heating and result in the formation of pores. To prevent foaming, the heating time should be prolonged, and the solvent should be evaporated slowly. This is uneconomical and undesirable.
Secondly, a method has been proposed which comprises coating a synthetic resin powder on the exposed metal portion of the welded part by electrostatic coating. The second method, however, has the defect that pores tend to form, and a problem is liable to arise in the film forming ability of the resin.
Thirdly, a method has been proposed which comprises providing a nozzle for injecting a molten resin in proximity to the exposed metal portion, moving the nozzle and a can body relative to each other, injecting the molten resin in a tape form, and successively adhering it to the exposed metal portion, thereby to organic-coat the welded part, as disclosed in U.S. Pat. No. 3,077,171, particularly FIG. 7 thereof. According to the third method, it is necessary to carry the molten resin as far as the tip of the welding roll electrode in a welding machine and form a film of the resin on the exposed metal portion so that it continuously has a uniform width and thickness. This requires a sophisticated technique, and involves difficult maintenance.
Fourthly, a method has been proposed which comprises providing a roll within a can body, and successively press-adhering a tape-like resin to the exposed metal portion thereby to organic-coat the welded part, as disclosed in U.S. Pat. No. 3,077,171, particularly FIG. 6 thereof, and Japanese Laid-Open Patent Publication No. 17224/1981. According to the fourth method, it is technically difficult to carry the tape-like resin to the tip of the welding roll electrode, for example, and to cut it to a length corresponding to the length of the can. In other words, the fourth method has the disadvantage that it is difficult to adjust the tape-like resin accurately to the desired size, and to apply it accurately to the desired position. This disadvantage is remarkable especially when the can body has a small inside diameter. Furthermore, since the fourth method normally uses rolls made of an elastic material such as rubber and having a restricted outside diameter, the rolls disadvantageously undergo marked wear. According to this method, the tape-like resin should be pressed by the roll under a relatively strong pressure along the surface of the welded part. This leads to the inherent practical defect that the tape-like resin extends in a zig-zag fashion and is not properly adhered, and the support used for supporting the roll shakes to give rise to various problems.
The fourth method further has the following problems.
Firstly, to adhere the tape-like resin to the welded part of the can body with a sufficient adhesion strength, it is necessary to heat the welded portion of the metallic can body to a relatively high temperature (for example, at least 100.degree. C. above the softening point) and to press the tape-like resin against the welded part. At this time, the tape-like resin is melted by the heat from the welded part and can flow so as to fill the stepped portion in the welded part. Since, however, the tape-like resin is usually still at a relatively high temperature (for example, at least 20.degree. C. above the softening temperature) when the welded part kept at such a relatively high temperature to secure adhesive strength adheres to the tape-like resin by a roll-like pressing member and they leave the pressing member, namely when the presssure between the tape-like resin and the welded part is released, bubbles are liable to form in the tape-like resin after releasing the pressure. At the organic coated welded part in which bubbles are so formed, desirable corrosion resistance is difficult to obtain.
Secondly, to prevent formation of bubbles at the tape-like resin, it is necessary to lower the heating temperature at the welded part so that the temperature of the tape-like resin at the time when the welded part and the tape-like resin leave the roll-like pressing member can be lowered to below the softening temperature of the tape-like resin plus 20.degree. C. However, satisfactory organic coating of the welded part cannot be obtained at this time because the strength of adhesion between the welded part of the can body and the tape-like resin is low and the tape-like resin cannot flow sufficiently to fill the stepped portion in the welded part.
Furthermore, when the welded part is pre-heated as above, variations in heating temperature occur in the organic coated welded portion of the can body, and tend to result in the formation of bubbles or in variations in adhesion strength.
Thirdly, in the method disclosed in the abovecited Japanese Laid-Open Patent Publication No. 17224/1981, after application of the tape-like resin, the welded part of the can body is primarily heated to a relatively low temperature that does not cause formation of the tape-like resin, and the tape-like resin is pressed against the welded part with rolls to adhere it preliminarily. Thereafter, the welded part of the can body is secondarily heated to cause the tape-like resin to flow so that it completely fills the stepped portion in the welded part and also to increase the strength of adhesion between the can body and the tape-like resin. However, since the tape-like resin is not pressed at the time of the secondary heating, abrupt heating or heating to high temperatures will cause formation of bubbles in the tape-like resin. For this reason, the secondary heating time becomes longer (for example, 10 to 15 seconds or longer), and other devices are required for temperature control.
On the other hand, because the third and fourth methods use an extruded resin or tape-like resin, they are better than the first and second methods in that (a) they can form a coated film which is relatively uniform and free from pores.
As stated above, however, these methods have various problems to be solved, which arise when a correction material such as a tape-like resin is applied to the welded part of the can body.
The present invention has been made in view of the aforesaid background.
It is an object of this invention to provide a process and an apparatus for producing a welded can body having a properly organic coated welded part, in which the exposed metal portion of the inside of the welded part of the welded can body and its vicinity is properly protected.
Another object of this invention is to provide a new process and apparatus for producing a welded can body having an organic coated welded part in which a tape-like resin is used.
Another object of this invention is to provide a process and an apparatus for producing a welded can body having an organic coated welded part, in which a tape-like resin is accurately adhered to the exposed metal portion without formation of bubbles in the resin.
Another object of this invention is to provide a process and an apparatus for producing a welded can body having an organic coated welded part in which a tape-like resin of an accurately adjusted size is applied to the exposed metal portion at an accurate position.
Another object of this invention is to provide a process and an apparatus for producing a welded can body having an organic coated welded part in which a tape-like resin is applied to the exposed metal portion within a short period of time.